Conventional optical devices of augmented reality, i.e. optical devices for displaying the combination of virtual images and background images, are usually made as a helmet or glasses and have optical systems in front of a user's eye or eyes. The optical devices of augmented reality are based on the combination of an eyepiece to enlarge an image formed by compacted LCD, LCoS, CRT or OLED displays, or other spatial light modulators and an image combiner based on, for example, a beam-splitting cube or a semitransparent plate. The eyepiece and the beam splitter are often made as a monolithic structure. The disadvantage of such devices is their big size and weight.
To eliminate the above-mentioned disadvantage, lightguides with different embedded structures can be used. One example is presented in U.S. Pat. No. 8,433,172 disclosing an optical guide provided with two faces, an injection section for entry of a light beam into the optical guide, an extraction section exiting the light beam. The extraction section comprises a plurality of microstructures located on a surface of one of the two faces of the optical guide. The microstructures are made as prisms having a calculated angle in order to send back the light beam at a given angle relative to the opposite surface enabling the exit of the light beam from the optical guide. The main drawback of the described device is brightness non-uniformity because of properties of prism geometry separated by flat surfaces.
The closest prior art for the present invention is an optical device described in U.S. Pat. No. 7,457,040. FIG. 12 shows the side view of a lightguide optical element included in this device. As shown, the lightguide optical element comprises a planar light-transmitting substrate 11 having at least two major surfaces 11a, 11b and edges, an optical means 12 for coupling light into the substrate 11 by total internal reflection and at least one partially reflecting means 13 located inside the substrate 11. The first reflecting surface of the optical means 12 is illuminated by a collimated light beam emanating from a light source (not shown) located behind the device. The reflecting surface reflects the incident light beam such that the light beam is trapped inside the substrate 11 by total internal reflection. After several reflections from the surfaces 11a, 11b of the substrate 11, the trapped light beam reaches the selectively reflecting surface of the means 13, which couples the light beam out of the substrate 11 into a viewer's eye 14. The main drawback of the described device is the necessity of expensive coatings with predefined reflectance and transmittance properties depending on an incidence angle.